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Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens
Author(s) -
Lehmann Anne,
Alaasar Mohamed,
Poppe Marco,
Poppe Silvio,
Prehm Marko,
Nagaraj Mamatha,
Sreenilayam Sithara P.,
Panarin Yuri P.,
Vij Jagdish K.,
Tschierske Carsten
Publication year - 2020
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201904871
Subject(s) - chirality (physics) , bent molecular geometry , lamellar structure , phase (matter) , crystallography , phase transition , liquid crystal , tilt (camera) , polar , ferroelectricity , materials science , helix (gastropod) , supramolecular chirality , chemical physics , condensed matter physics , chemistry , dielectric , crystal structure , supramolecular chemistry , geometry , organic chemistry , physics , mathematics , snail , ecology , chiral symmetry breaking , composite material , biology , quark , quantum mechanics , nambu–jona lasinio model , optoelectronics , astronomy
A series of bent‐shaped 4‐cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short‐pitch heliconical lamellar liquid‐crystalline phase with incommensurate 3‐layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric‐(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field‐induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non‐tilted and ferreoelectric switching lamellar phase, providing an alternative non‐chiral way for the transition from anticlinic to synclinic tilt.